1. The Field of the Invention
The invention generally relates to integrated VCSEL and photodiode combinations. More specifically, the invention relates to methods and apparatuses for reducing the effects of spontaneous emissions from a VCSEL on an integrated photodiode.
2. Description of the Related Art
Lasers have become useful devices with applications ranging from simple laser pointers that output a laser beam for directing attention, to high-speed modulated lasers useful for transmitting high-speed digital data over long distances, to sensors for determining speed, distance, material compositions and the like. Several different types of lasers exist and find usefulness in applications at the present time.
One type of commonly used laser is known as a vertical cavity surface emitting laser (VCSEL). A VCSEL is formed in part by forming a first mirror from Distributed Bragg Reflector (DBR) semiconductor layers on a semiconductor wafer substrate. The DBR layers alternate high and low refractive indices to create the mirror effect. This method creates a mirror that reflects over 99% of on-axis emissions.
An active layer is then formed on the first mirror. The active layer includes a number of quantum wells for stimulating the emission of laser energy. The active layer includes a pn semiconductor junction. It is in the active layer that electrons switching from the conduction band to the valance band produce photons. Below a threshold current for a given VCSEL, light is emitted spontaneously. When a laser is biased by a current above the given threshold, laser emissions are more prevalent, although spontaneous emissions continue to be a portion of the active layer output. Spontaneous emissions tend to be at a number of different wavelengths and tend to radiate isotropically (equally in all directions) from the active layer. Laser emissions tend to be in a very narrow wavelength band centered around a frequency for which the VCSEL was designed and tend to radiate axially in the direction of the vertical axis of the VCSEL. Notably while the VCSEL and vertical axis are used herein and connote a particular orientation, those of skill in the art will appreciate that the vertical axis can be positioned in a horizontal axis, or any other orientation. Vertical, as used herein, simply refers to the axis along which epitaxial layers (as described in more detail below) are formed.
A second mirror is formed on the active layer using more DBR semiconductor layers. Thus the VCSEL laser cavity is defined by top and bottom mirrors which cause a laser beam to be emitted from the surface of the laser. The second mirror has a reflectivity of over 98–99.5% for on axis emissions.
As alluded to above, a VCSEL is typically forward biased by a current. Forward biasing involves connecting a higher potential (voltage) source at the anode (near the p type material of the pn junction) while a lower potential source is connected at the cathode (near the p type material of the pn junction) of the VCSEL. Currents through the VCSEL above a threshold current cause laser emissions from the active layer.
In some simple applications, the lasers may be operated open loop. I.e., the lasers do not require feedback, or can operate satisfactorily without feedback. For example, in most laser pointer applications, the output power of the laser beam may be controlled without reference to the actual output power. In other applications, it may be very important to precisely gauge the amount of actual output power emitted by the laser while it is operating. For example, in communications applications it may be useful to know the actual output power of the laser such that the output power of a laser may be adjusted to comply with various standards or other requirements. Additionally, in sensor applications, it is useful to gauge the effects of conditions external to the VCSEL by monitoring the VCSEL output power.
Many applications use a laser in combination with a laser power monitoring photodiode or other photosensitive device. A photodiode has current characteristics that change as light impinges the diode. The photodiode either has no bias or is implemented in a reverse bias configuration such that the cathode is connected to a higher potential while the anode is connected to a low voltage or ground. In a photodiode in the reverse biased or unbiased configuration, current is generated within the photodiode as light impinges the photodiode.
An appropriately placed photodiode may be used as one element in the feedback circuit for controlling the laser. Photodiodes are typically fabricated of the same or similar semiconductor materials as VCSEL diodes. Recent technology therefore, has focused on implementing a photodiode and VCSEL diode monolithically together on the same substrate. Exemplary fabrications include epitaxially forming a photodiode on a substrate followed by forming a VCSEL on top of the photodiode. Other fabrications include forming a VCSEL on one side of a wafer substrate and the photodiode on the other side of the wafer substrate. Still other fabrications include forming a VCSEL on a substrate followed by forming a photodiode on top of the VCSEL. Additionally, the photodiode may be placed within a mirror that is part of the VCSEL.
One challenge that arises when a VCSEL and photodiode are formed together monolithically on a substrate relates to photons caused by spontaneous emissions being received by the photodiode. A VCSEL has photon emissions caused by spontaneous emissions and laser emissions. As described above, spontaneous emissions are typically undesirable emissions that are emitted from the active layer of the VCSEL. Spontaneous emissions may be at a variety of wavelengths. Laser emissions are typically emitted axially along the vertical axis from the active layer of the VCSEL at the wavelength for which the VCSEL was designed.
Photons emitted due to spontaneous emission often couple to photodiodes formed monolithically with a VCSEL better than photons caused by laser emissions. Illustratively, an axially directed photon, such as laser emission typically predominately include, will likely be reflected by a bottom DBR mirror before reaching a photodiode formed under the bottom DBR mirror. As previously stated herein, on axis emissions are reflected with more than 99% efficiency. On the other hand, photons caused by spontaneous emission that are off axis and directed toward the photodiode, will more easily pass through the bottom DBR mirror to the photodiode. Additionally, spontaneously emitted photons that are directed away from the bottom mirror and the photodiode will likely be, at least partially, reflected towards the photodiode.
Total internal reflection occurs when a light beam encounters an interface of two materials with different refractive indices at an angle at or above a critical angle. Total internal reflection will occur in a VCSEL at the VCSEL/air interface at the top of the VCSEL. At this interface, the critical angle is about 15°, which results in a large percentage of the light being totally internally reflected. An interface with different refractive indices also exists at the interface between the active layer and the bottom mirror. The critical angle at this interface is 60–70°, which results in a large percentage of light traveling towards the bottom mirror being passed through the bottom mirror to the photodiode. Therefore, a large amount of the spontaneous emissions will be directed at one time or another towards the photodiode. Further, the majority of photons directed towards the edges of a VCSEL will be totally internally reflected such that they are likely to impinge the photodiode.
The photons caused by spontaneous emission, in one worst case scenario, can completely swamp the signal caused by laser emissions. The photons caused by spontaneous emission cause difficulties in determining the correct amount of laser energy emitted by laser emission from a VCSEL.
It would therefore be useful to minimize the amount of spontaneously emitted photons reaching a photodiode integrated with a VCSEL.